Disclosed herein are gas heaters for gas chromatography convection ovens, methods for applying such heaters in gas chromatography systems and gas chromatography systems having additional gas heaters for enhanced temperature programming performance.
In gas chromatography the components of a mixture are separated as volatilized components and carried through a separation column to a detector. The separation column is typically a long capillary suspended in a convection oven. Many gas chromatographic applications require the oven to be heated in a programmed manner during the analysis.
FIG. 1 illustrates the space in a convection oven 1 useful in gas chromatography. The oven space is typically divided by a baffle 5 into a heater space 3 containing a heater element 6 and a fan 2, and a column space 4 containing one or more separation columns (not shown). The baffle typically has a central bore allowing air to be drawn into the fan from the column space into the heater space. The baffle is also designed to permit flow from the heater space to the column space between the baffle periphery and the walls of the oven to permit a circulating flow of forced hot gas, typically air. Electrical power is applied to a heater 6 (e.g. resistively heated metal wire) during the heating cycle from a power supply 9 connected to a source of electric power 10. A temperature control circuit 7 receives a signal indicative of the oven temperature from a sensor 8 and sends a control signal to a power supply 9.
It is sometimes desirable to minimize the time required to analyze a sample. The time required to analyze a sample by gas chromatography depends on many factors including the heating rate that the oven can achieve. The heating performance of a gas chromatography oven is affected by (a) the power of a heater, (b) the characteristics of the heat transfer medium (typically air), (c) and the rate of heat loss to the surroundings. Electrical power is supplied to commercial gas chromatography from a single electrical power source, for example the power supply is connected to 100 volt (V), 20 amp or 220 V, 20 amp service. Therefore, the ability to generate heat energy is limited by the available electrical power. The heating power is limited by the amount of electrical current that can be conducted through the wire, which is limited by the electrical source, and the physical properties of the wire. Table 1 shows the heating performance of a typical gas chromatography.
A temperature heating profile for a gas chromatographic oven is illustrated in FIG. 2. The solid line represents the xe2x80x9ctrue maximum heating ratexe2x80x9d which is the heating rate for the oven when full power is applied to the heater. Because a controlled heating rate is usually desired, in practice the instrument is usually limited to selected heating rate values including a xe2x80x9cmaximum selectable heating ratexe2x80x9d (as indicated by the dashed line) which is the maximum heating rate that can be selected for a particular gas chromatograph. As such, the maximum selectable heating rate is an artificial limit imposed by design.
Gas chromatograph ovens can operate at the maximum selectable heating rate over a limited temperature range, depending upon the allowed values and the heating performance of the oven. At low temperatures there is sufficient heating power available to overcome the heat loss to the surroundings and the true maximum heating rate exceeds the maximum selectable heating rate. Under these conditions the gas chromatograph oven can achieve the maximum selectable heating rate. As the temperature of the oven rises, however, the true maximum heating rate decreases such that the maximum selectable heating rate can no longer be achieved. This occurs at a temperature less than that represented by the intersection of the two lines shown in FIG. 2. The gas chromatographic oven cannot achieve the maximum selectable heating rate above the upper performance temperature. This condition results in uncontrolled operation of the gas chromatograph. Results obtained under such conditions are less reliable compared to results obtained under controlled heating conditions. Accordingly, the heating rate is often reduced below the maximum selectable heating rate when the oven is heated above the upper performance temperature.
Gas chromatographs having temperature-programmable ovens have been commercially available for more that 30 years. Temperature programming of gas chromatography systems is important for facilitating the separation of mixtures of components. Unique heater systems for gas chromatography systems are known in the art. See, for instance, U.S. Pat. Nos. 3,043,127; 3,057,183; 3,165,146; 3,225,520; 3,581,465; 4,050,911; 4,923,486; 5,028,243; 5,215,556; 5,544,276; 5,547,497; 5,744,029; 5,807,426; 5,830,353, all of which are incorporated herein by reference in their entireties.
Despite the great variation in heating systems, modern gas chromatography apparatus is deficient in its ability to maintain high programmed rates of heating over an extended temperature range, e.g. 70xc2x0 C. per minute at temperatures much over 115xc2x0 C. when supplied with 110 volt 20 amp service.
The desire to achieve faster heating rates is evidenced by design improvements witnessed throughout their development. Attempts to improve their heating performance include reducing the thermal mass associated with gas chromatographic ovens thereby reducing the power required to heat the oven and the heat loss to the surroundings. The heating performance of the oven heater has been improved. Circulation of the heat exchange medium has been improved by the proper placement of fans and the design of baffles. Some gas chromatographs (Model 222 Perkin-Elmer Corp., Norwalk, Conn. USA) also employed resistively heated metal tubes containing a separation medium to achieve better heating performance.
More recently, faster heating has been achieved by passing an electrical current through an electrically conductive coating on the outside of the separation column (Jain, V. and Phillips, J. (1995); J. Chrom. Sci.33:55.). Another technique employs resistively heated wires wrapped around the separation column (Ehrmann, E. et. al. (1996); J. Chrom. Sci. 34:533). A commercial instrument (Flash GC, Thermedics Detection Inc. Chelmsford, Mass.) uses electrical current to heat a low thermal mass metal sheath containing a separation column.
Modification kits have also become commercially available that permit faster heating of the separation column in existing gas chromatographs. An oven insert kit (PN G2646A, Agilent Technologies, Wilmington, Del. USA) is comprised of an insulating pillow that is inserted into the column space to reduce the volume of the oven. A kit in which the separation column is located within a thin metal sheath that can be rapidly heated is also available (EZ Flash GC accessory, Orion Research, Beverly, Mass. USA).
An object of this invention is to provide improvements in the heating performance of gas chromatography systems to permit high heat rates of thermal programming at elevated temperatures.
This invention provides gas chromatography systems with improved temperature programming capability due to the addition of at least one additional electrically powered gas heater. Such heaters can be provided in a kit comprising means for mounting the heater in a convection oven to promote even distribution of convective heat from the heater to the separation column. The means for mounting the heater preferably also comprises a radiant heat shield between said heaters and separation columns.